In recent years, electronic devices, particularly portable devices such as mobile phones PDAs (personal digital assistants), and notebook personal computers, are being reduced in size, weight, and thickness and are becoming more powerful, and such portable devices are becoming widespread. As the range of use of these portable devices expands, batteries for driving the portable devices are becoming very important components. Among the batteries, nonaqueous electrolyte secondary batteries, typified by lithium ion secondary batteries, having high energy density and high capacity are widely used.
Generally, such nonaqueous electrolyte secondary batteries are produced as follows. A negative electrode containing a negative electrode active material composed of a carbon material and the like that can store and release lithium ions and a positive electrode containing a positive electrode active material composed of a lithium-containing transition metal composite oxide (such as LiCoO2, LiNiO2, or LiMn2O4) are formed into sheets on the surfaces of metal foils serving as collecting substrates (collectors). A sheet-shaped positive electrode and a sheet-shaped negative electrode are thereby obtained. The sheet-shaped positive electrode and the sheet-shaped negative electrode are wound or laminated through separators formed also into sheets and are stored in a case. Each of the sheet-shaped positive electrodes and the sheet-shaped negative electrodes has a structure comprising metal foil serving as a collecting substrate (collector) and a mixture layer containing an active material and formed on the surface of the metal foil. Such an electrode can be formed by coating a collector material with a negative electrode active material slurry (or paste) or positive electrode active material slurry (or paste) and drying it.
The negative electrode active material slurry (paste) contains a binder in addition to a negative electrode active material composed of a carbon material and the like that can store and release lithium ions. A binder for a negative electrode including styrene/butadiene latex (SBR) as a main component is disclosed in Patent document 1 (Japanese Patent Application Laid-Open No. Hei 5-74461) as a binder.
In Patent document 1, carboxymethylcellulose used as a water-soluble thickener is dissolved in water to prepare an aqueous solution, and SBR and a negative electrode active material are mixed into the aqueous solution to produce a slurry. The slurry used as a coating solution is applied to a substrate and dried to form a sheet-shaped negative electrode.
To produce the positive electrode of a nonaqueous electrolyte secondary battery, an organic solvent such as N-methyl-2-pyrrolidone (NMP) is conventionally used as a solvent. However, water is recently being used as the solvent to reduce handling cost and take into account the influence on the environmental load of the solvent discharged.
The positive electrode active material slurry (paste) contains a binder in addition to a conducive material such as carbon and a lithium-containing transition metal composite oxide (such as LiCoO2, LiNiO2, or LiMn2O4) serving as the positive electrode active material. Patent document 2 (Japanese Patent Application Laid-Open No. 2003-157847) describes cellulose of which 1% aqueous solution has a viscosity of 4,000 mPa·s or more, such as carboxymethylcellulose, as a binder. Patent document 2 describes that an active material paste is prepared by adding the carboxymethylcellulose together with a conductive material, polytetrafluoroethylene (PTFE), and other materials to pure water.
In a method for coating a collecting substrate with the negative electrode active material slurry (paste) or positive electrode active material slurry (paste), a doctor blade spaced apart by a predetermined distance from the surface of the collecting substrate to be coated is used to draw a layer of slurry in an amount corresponding to the gap between the doctor blade and the collecting substrate, and a sheet-shaped electrode plate is thereby formed (for example, as disclosed in Patent document 3 (Japanese Patent Application Laid-Open No. Hei 4-242071)).
In another proposed method, a coating solution of an electrode material is discharged and applied onto a running collecting substrate wound around a back-up roll using an extrusion-type injector with a slot nozzle (for example, as disclosed in Patent document 4 (Japanese Patent Application Laid-Open No. Hei 7-65816)).